When a speech/audio signal is transmitted in a packet communication system typified by Internet communication, a mobile communication system, or the like, compression/coding technology is often used in order to increase speech/audio signal transmission efficiency. Furthermore, there is a growing demand for a technology of not simply encoding a speech/audio signal at a low bit rate but also encoding a wider band speech/audio signal in recent years.
In response to such a demand, various band extension technologies are being developed which encode a wideband speech/audio signal without drastically increasing the amount of coded information. For example, a technology is disclosed which applies gain information in a linear region and gain information in a logarithmic domain to spectrum data in a low-frequency part out of spectrum data obtained, for example, by converting an input audio signal corresponding to a certain time to generate spectrum data in a high-frequency part (see Patent Literature 1 and Non-Patent Literature 1). Furthermore, hierarchy coding schemes which encode a wideband signal in a hierarchical manner have been developed so far. For example, Non-Patent Literature 2 discloses a technology of encoding a wideband signal using a hierarchy coding scheme made up of five layers.